The invention concerns an integrated inorganic/organic complementary thin-film transistor circuit comprising a first and a second transistor which are operatively connected and provided on a common substrate, wherein the first transistor is an inorganic thin-film transistor and the second transistor an organic thin-film transistor, wherein the separate gate electrodes are provided for each of the transistors, and wherein the complementary thin-film transistor circuit forms a multilayer thin-film structure.
The present invention also concerns a method for fabricating an integrated inorganic/organic complementary thin-film transistor circuit comprising a first and a second transistor which are operatively connected and provided on a common substrate, wherein the first transistor is an inorganic thin-film transistor and the second transistor a organic thin-film transistor, and wherein the complementary thin-film transistor circuit forms a multilayer thin-film structure with successively deposited and patterned thin-film layers, and wherein the method comprises depositing separate gate electrodes for respectively the first and the second transistor on a common substrate, and depositing material for the source electrode and the drain electrode of the organic thin-film transistor on the same level in the thin-film structure of the organic thin-film transistor.
Finally the present invention concerns a method for fabricating an integrated inorganic/organic complementary thin-film transistor circuit, comprising a first and a second transistor which are operatively connected and provided on a common substrate, wherein the first transistor is an inorganic thin-film transistor and the second transistor an organic thin-film transistor, and wherein the complementary thin-film transistor circuit forms a multilayer thin-film structure with successively deposited and patterned thin-film layers.
Integrated circuits of silicon realized as complementary metal-oxide semiconductors dominate the markets for a number of microelectronic applications such as microprocessors. But complementary circuits may also be of interest for more general application, e.g. in portable battery-operated electronic products, as they can provide very low static power dissipation for digital circuits. It has, however, turned out to be difficult to realize complementary integrated thin-film circuits with sufficient performance for commercial applications.
Hydrogenated thin-film transistors of silicon (a-Si:H TFT) have found a new application in thin-film components, particularly in liquid crystal displays with active matrix. However, complementary a-Si:H circuits are problematic, as the hole transport mobility typically is much lower than the electron transport mobility. Recently TFTs with organic active layers have been fabricated and with performance comparable to that which can be obtained with amorphous silicon devices (a-Si:H devices).
For instance there is in U.S. Pat. No. 5,347,144 (Garnier and al.) disclosed a thin-film field-effect transistor with an MIS structure which includes a thin semiconductor layer between the source and drain electrode. The thin semiconductor layer contacts a surface of a thin-film made of isolating material which at its second surface contacts a conducting grid. The semiconductor is made of at least one polyconjugated organic compound with a determined molecular weight. As organic semiconductor material Garnier and al. among others mention different various aromatic polycyclic hydrocarbons and among these polyacenes. The transistor of Garnier and al. is stated to be particularly suited as a switching or amplifying device.
Also simple organic complementary thin-film transistor circuits have been discussed in the literature, but have not shown the desired performance properties. Further attempts have been made building complementary circuits with combinations of inorganic and organic devices on separate substrates and with external connection.
In U.S. Pat. No. 5,625,199 (Baumbach and al.) there is, however, disclosed a complementary circuit with an inorganic n-channel thin-film transistor and an organic p-channel thin-film transistor. The n-channel thin-film transistor employs hydrogenated amorphous silicon as active material and the p-channel of the organic thin-film transistor employs xcex1-hexathienylene (xcex1-6T) as active semiconductor material. The complementary thin-film transistor circuit according to Baumbach and al. can be used for implementing an integrated complementary inverter or other complementary circuits.
The integrated complementary inorganic/organic thin-film transistor according to Baumbach and al. is, however, encumbered with a number of disadvantages both from a processual point of view as well as with regard to general application in more comprehensive transistor circuits. Thus Baumbach and al. propose to provide respectively the source and drain electrodes on both sides of the organic semiconductor layer, something which firstly is not necessary and additionally comports a number of disadvantages in the fabrication. Further the source and drain contacts of the organic thin-film transistor must be formed in different steps and it will also be difficult to pattern contacts on the top of the organic semiconductor unless shadow masks are used.
Nor has the complementary thin-film transistor according to Baumbach an isolated organic semiconductor material in the organic thin film transistor. As it will be desirable to be able to turn the inorganic transistor on and to turn the organic transistor off or vice versa using potential with the same sign, this may be problematic. In the complementary thin-film transistor according to Baumbach and al. it is probable that an undesirable large leakage will be problematic if the complementary thin-film transistor shall be used in complex circuits. An inverter realized according to Baumbach and al. switches as stated in the cited U.S. patent at about 5 V at a supply voltage of 7.2 V. Another disadvantage of the complementary thin-film transistor according to Baumbach and al. is that a common gate electrode is used both for the n-channel and the p-channel transistor. More complex transistor circuits built from complementary devices shall require that common electrodes are not used in these. Even in simple inverters a common gate electrode will give increased stray capacitance. Further it shall be remarked that the complementary thin-film transistor according to Baumbach and al. uses the inorganic transistor as n-channel transistor and the organic transistor as p-channel transistor, something which is understandable in light of the materials proposed. It is, however, evident from Baumbach and al. that the use of organic materials which may be used for forming active semiconductors of the n-type demands relatively complicated and costly fabricating processes and hence is not easy to realize for the time being.
In U.S. Pat. No. 5,162,228 (Shieh and al.) there is disclosed a method of fabricating a thin-film transistor with separate gate electrodes. The inorganic and organic thin-film transistors are of the n type and p type respectively and are shown integrated into a complementary circuit with the source and drain electrodes in both transistors being on the same respective levels. The organic thin-film transistor is shown patterned with an active semiconductor material of the p type and it is evident that the problem with stray capacitance essentially may be avoided. Shieh and al. relies on low-temperature processes in the deposition of the active semiconductor materials, but does not consider the problem inherent in the conventional patterning methods applied to an organic semiconducting material. Neither is the device as disclosed by Shieh and al. amenable to embodiments with n-type organic materials, particularly as the metal electrodes are deposited on the top of the n-type transistor""s active and patterned semiconductor material.
A first object of the present invention is hence to overcome the disadvantages which are connected with prior art and particularly to provide an integrated complementary inorganic/organic thin-film transistor circuit which is suited for use in large transistor circuits. Another object is to provide complementary thin-film transistor circuits which allow a cheap fabrication and simultaneously have low static power consumption, such that they can be used in portable battery-operated equipment.
A further object of the present invention is to provide an uncomplicated and inexpensive method for fabricating integrated complementary inorganic/organic thin-film transistor circuits and this in as few process steps as possible, while a device with good electric properties is obtained and whereby it particularly shall be possible to realize the inorganic transistor as an n-channel transistor and the organic transistor as a p-channel transistor or vice versa.
The above-mentioned and other objects are achieved according to the invention with an integrated inorganic/organic complementary thin-film transistor circuit which is characterized in that the inorganic thin-film transistor is an n-channel transistor and that the organic thin-film transistor is a p-channel transistor, or vice versa, the organic active transistor material in each case being respectively a p-channel organic semiconductor material or an n-channel organic semiconductor material, that the organic semiconductor material is provided in a substantially global layer over at lease one suitably patterned isolating layer on the top of the circuit, such that said isolation layer or layers is/are broken in the region of the second transistor, whereby the organic active semiconductor in the second transistor in any case contacts the source and drain electrodes thereof and is provided in complete electrical isolation against the first transistor.
According to the invention the inorganic active semiconductor material is advantageously selected among hydrogenated amorphous silicon (a-Si:H), hydrogenated or unhydrogenated microcrystalline silicon (xcexcc-Si:H;xcexcc-Si), hydrogenated or unhydrogenated polycrystalline silicon (pc-Si:H;pc-Si), single crystal silicon, copper-doped polycrystalline germanium (pc-Ge:Cu), cadmium selenide (CdSe), cadmium telluride (CdTe), or composite inorganic semiconductors based on said materials, possibly in single crystal form.
Where the inorganic thin-film transistor is an n-channel transistor, the inorganic active semiconductor material is preferably amorphous silicon (a-Si:H), and where the inorganic transistor is a p-channel transistor, the inorganic active semiconductor material is preferably a p-channel silicon material, particularly p-channel hydrogenated amorphous silicon (a-Si:H).
In an advantageous embodiment the active semiconductor material in the inorganic thin-film transistor comprises at least one polyconjugated organic compound with a specific molecular weight. It is then advantageous that the polyconjugated organic compound or compounds are selected selected among conjugated oligomers, polycyclic aromatic hydrocarbons, particularly polyacenes, or polyenes.
Where the organic thin-film transistor is a p-channel transistor, it is advantageous that the organic active semiconductor material is pentacene, and where the organic thin-film transistor is an n-channel transistor, it is advantageous that the organic active semiconductor material is copper hexadecafluorophtalocyanide.
Finally, it is according to the invention particularly advantageous that the source electrode and the drain electrode of the organic thin-film transistor is provided in one and the same level in the thin-film structure of the organic thin-film transistor.
A first method for fabricating an integrated inorganic/organic thin-film transistor circuit is according to the invention characterized by forming the inorganic thin-film transistor as an n-channel transistor and the organic thin-film transistor as a p-channel transistor by depositing respectively an n-channel inorganic active semiconductor material and a p-channel organic active semiconductor material or correspondingly forming the organic thin-film transistor as an n-channel transistor and the inorganic thin-film transistor as a p-channel transistor by depositing respectively an n-channel organic active semiconductor material and a p-channel inorganic active semiconductor material, providing in any case at least one global isolating layer over first transistor and broken in the region of the second transistor by a suitable patterning to expose the source and drain electrodes and the gate isolator of the second transistor, and providing the organic active semiconductor material in a global layer on the top of the isolating layer or layers and such that it covers the exposed portion of the second transistor, whereby the active organic semiconductor in the second transistor is provided contacting the source and drain electrodes thereof and in complete electrical isolation against first transistor by a re-entrant edge of the broken profile of the isolating layer or layers.
A second method for fabricating an integrated inorganic/organic complementary thin-film transistor circuit is according to the invention characterized by comprising steps for depositing separate gate electrodes of a first metal for each of the two transistors on a common substrate, depositing separate inorganic isolators of silicon nitride (SiNx) over each gate electrode, depositing an inorganic active semiconductor in the form of hydrogenated amorphous silicon (a-Si:H) above one of the gate electrodes which thus forms the gate electrode of the first transistor, depositing and patterning an n+ doped layer of either hydrogenated amorphous silicon (n+a-Si:H) or hydrogenated microcrystalline silicon (n+ xcexcc-Si:H) or hydrogenated polycrystalline silicon (n+pc-Si:H) as source and drain contacts for the first transistor, depositing and patterning the source and drain electrodes of the first transistor in form of a second metal over the source and drain contacts thereof, depositing and patterning the source and drain electrodes for the second transistor in the form of a third metal in the same layer level in the thin-film structure, forming an isolating double layer over the whole organic thin-film transistor and patterning this such that the source and drain electrodes and the gate isolator in the second transistor become exposed, whereafter a layer of pentacene is deposited above the isolating double layer and the exposed portion of the second transistor, the pentacene layer in the exposed portion forming the active semiconductor material of the organic thin-film transistor and being provided electrically isolated against the additional pentacene layer broken by a re-entrant edge of the profile of the isolating double layer.
In an advantageous embodiment of the last-mentioned method according to the invention the steps for forming the inorganic thin-film transistor are realized in a tri-layer process which forms an inverted staggered three-layer structure.
In another advantageous embodiment of the last-mentioned method according to the invention the steps for forming the inorganic thin-film transistor are realized in a back-channel etch process.
In an advantageous embodiment of the last-mentioned method according to the invention the active semiconductor in the form of pentacene in the organic thin-film transistor is isolated by a re-entrant profile of a broken double layer of polymethylmetacrylate (PMMA) and Novolac photoresist.
In an advantageous embodiment of the last-mentioned method according to the invention gold is evaporated thermally for forming the source and drain electrodes of the organic thin-film transistor.
Finally, the pentacene layer which is deposited over the isolating double layer can be removed.